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Use SILK VBR when using CBR with DRED

DRED will absorb the bitrate variation

--- a/src/opus_encoder.c

+++ b/src/opus_encoder.c

@@ -1844,7 +1844,14 @@

         }

         if (st->silk_mode.useCBR)

         {

+           /* When we're in CBR mode, but we have non-SILK data to encode, switch SILK to VBR with cap to

+              save on complexity. Any variations will be absorbed by CELT and/or DRED and we can still

+              produce a constant bitrate without wasting bits. */

+#ifdef ENABLE_DRED

+           if (st->mode == MODE_HYBRID || dred_bitrate_bps > 0)

+#else

            if (st->mode == MODE_HYBRID)

+#endif

            {

               /* Allow SILK to steal up to 25% of the remaining bits */

               opus_int16 other_bits = IMAX(0, st->silk_mode.maxBits - st->silk_mode.bitRate * frame_size / st->Fs);

-- 

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